1. Field of the Invention
The present invention relates to an image sensor and a method of fabricating the same and more particularly, to an image sensor comprising photoelectric converter elements formed on a transparent substrate, each of the elements having a photoelectric converting layer and lower and upper electrodes located at lower and upper sides of the layer, and a method of fabricating the sensor. The sensor is preferably used for facsimiles, image scanners, and so on.
2. Description of the Related Art
Image sensors are incorporated into various image sensing devices or apparatuses (e.g., facsimiles and image scanners) to detect or sense the light reflected by the surface of an object to be sensed such as a paper document. Image sensors of this type sense the image on an object linearly and thus, they have a typical structure that photodiodes serving as photoelectric converter elements and Thin-Film Transistors (TFTs) serving as switching elements are arranged along a straight line. An example of the typical structure is shown in FIGS. 1 to 4.
As shown in FIG. 1, a prior-art image sensor 100 comprises a signal line 110 extending along a specified direction (which is defined as the X direction here), photodiodes 112 arranged at regular intervals along the line 110 in the X direction, TFTs 113 arranged at regular intervals along the line 110 in the X direction. The line 110, the photodiodes 112 and the TFTs 113 are formed on a transparent substrate 101.
One end of the signal line 110 extends in a direction perpendicular to the X direction (which is defined as the Y direction here) to be connected to a pad 120 for connecting electrically the line 110 to an external circuit or device. The pad 120 is approximately square.
Each of the photodiodes 112 is formed in their pixel area 121 with an approximately square shape. As explained later, the pixel areas 121 are defined by a patterned amorphous silicon (a-Si) layer that forms the photodiodes 112. Each of the TFTs 113 is located near the corresponding photodiode 112.
Each of the pixel areas 121, an adjoining, corresponding one of the photodiodes 112, and an adjoining, corresponding one of the TFTs 113 form the pixel of the sensor 100. Thus, it is said that the sensor 100 comprises the pixels aligned regularly in the X direction. Since all the pixels have the same structure, the structure of one pixel is explained below for simplification.
FIGS. 2 to 4 are cross-sectional views along the lines IIxe2x80x94II, IIIxe2x80x94III, and IVxe2x80x94IV in FIG. 1, respectively, which show the detailed pixel structure of the prior-art sensor 100.
As shown in FIG. 4, a patterned semiconductor layer 130 is formed on the upper surface of the transparent substrate 101. The layer 130 is approximately rectangular in plan shape. The layer 130 is selectively doped with impurity, forming a pair of source/drain regions 131a and 131b of the TFT 113. The undoped part of the layer 130 between the source/drain regions 131a and 131b forms a channel region 132. An electrically conductive channel of the TFT 113 is generated in the region 132 on operation.
A dielectric layer 102 is formed to cover the whole surface of the substrate 101. The layer 102 covers the semiconductor layer 130 also. The part of the layer 102 located on the channel region 132 serves as the gate dielectric of the TFT 113.
A gate electrode 133 is formed on the part of the dielectric layer 102 serving as the gate dielectric. The electrode 133 is located just over the channel region 132.
A first interlayer dielectric layer 104 is formed on the dielectric layer 102 to cover entirely the same. The layer 104 covers the gate electrode 133 as well.
As shown in FIG. 2, the lower electrode 105 of the photodiode is formed on the first interlayer dielectric layer 104. The electrode 105, which has an approximately square plan shape, includes a connection part 105a that is used for electrical connection to the source/drain region 131b. The connection part 105a is formed to be approximately rectangular and to extend to the region 131b.
An amorphous silicon (a-Si) layer 106 with an approximately square plan shape is formed on the lower electrode 105 of the photodiode 112. This a-Si layer 106 defines the pixel area 121 of the sensor 100.
A transparent, upper electrode of the photodiode 112, which has an approximately square plan shape, is formed on the a-Si first interlayer dielectric layer 104 to cover entirely the underlying a-Si layer 106. The upper electrode 107 includes a connection part 107a that is used for electrical connection to the signal line 110. The connection part 107a is formed to be approximately rectangular. The part 107a extends to be overlapped with the line 110.
As shown in FIGS. 2 and 3, a patterned barrier metal layer 108 is formed on the connection part 107a of the upper electrode 107. The layer 108 serves to prevent the substance contained in the signal line 110 from diffusing into the upper electrode 107.
A second interlayer dielectric layer 109 is formed on the first interlayer dielectric layer 104, covering the lower electrode 105, the upper electrode 107, and the barrier metal layer 108.
As shown in FIGS. 2 to 4, the signal line 110 and two wiring lines 134 and 135 are formed on the second interlayer dielectric layer 109. The signal line 110 overlaps with the underlying connection part 107a of the upper electrode 107 and the underlying barrier metal layer 108. The line 110 is contacted with the layer 108 and electrically connected to the same (and the electrode 107) by way of contact holes 118 of the dielectric layer 109.
One end of the wiring line 134 is contacted with the underling connection part 105a of the lower electrode 105 of the photodiode 112 and electrically connected to the same by way of contact holes 136 of the second interlayer dielectric layer 109. The other end of the line 134 is contacted with the underling source/drain region 131b and electrically connected to the same by way of contact holes 137 that penetrate the underlying dielectric layer 102 and the first and second interlayer dielectric layers 104 and 109. Thus, the wiring line 134 interconnects the lower electrodes 105 with the source/drain region 131b of the TFT 113.
One end of the line 135 is contacted with the underling source/drain region 131a and electrically connected to the same by way of contact holes 138 that penetrate the underlying dielectric layer 102 and the first and second interlayer dielectric layers 104 and 109.
The combination of the pair of source/rain regions 131a and 131b, the channel region 132, the dielectric layer 102, and the gate electrode 133 constitutes the TFT 113. The combination of the lower electrode 105, the a-Si layer 106 and the upper electrode 107 constitute the photodiode 112 and its capacitor (not shown) for storing the electrical charge to be generated in the photodiode 112.
Next, the operation of the prior-art image sensor 100 is explained below.
When incident light enters the photodiodes 112, electrical charges are generated in the photodiodes 112 and stored temporarily in their capacitors. The charges thus stored in the capacitors are sequentially read out and outputted as electrical signals by sequentially driving the TFTs 113 serving as the switching elements. The driving operation of the TFTs 113 are typically carried out at a rate of several hundreds kilohertz (kHz) or several hundreds megahertz (MHz).
The prior-art image sensor 100 is fabricated in the following way.
First, a polysilicon layer (not shown) is formed on the upper surface of the transparent substrate 101. The substrate 101 is made of transparent glass for incident light, for example. The polysilicon layer is patterned to have a predetermined shape, forming the patterned semiconductor layer 130 for the TFTs 113 on the substrate 101. Silicon dioxide (SiO2) is deposited on the substrate 101 to entirely cover its surface and the semiconductor layer 130, forming the dielectric layer 102 on the substrate 101. The gate electrode 133 is selectively formed on the dielectric layer 102 to be overlapped with the semiconductor layer 130. Following this, specific impurity is selectively doped into the semiconductor layer 130, forming the pair of source/drain regions 131a and 131a therein. The undoped part of the layer 130 forms the channel region 132.
Subsequently, SiO2 is deposited to cover the entire surface of the substrate 101, forming the first interlayer dielectric layer 104 that covers the whole substrate 101. A metal layer (not shown) is formed on the dielectric layer 104 and is patterned to have a predetermined plan shape, forming the lower electrode 105 including the connection part 105a. For example, the metal layer is made of chromium (Cr).
An a-Si layer (not shown) with a thickness of approximately 1 xcexcm is formed on the first interlayer dielectric layer 104 and then, it is patterned to have a predetermined plan shape, forming the patterned a-Si layer 106. Through this patterning process, the layer 106 is divided into islands. These islands of the layer 106 define the square pixel areas 121, as shown in FIG. 1. The four sides of each area 121 are approximately 50 to 100 xcexcm.
A transparent, electrically conductive layer (not shown) is formed on the first interlayer dielectric layer 104 to cover the whole a-Si layer 106. For example, the electrically conductive layer is made of Indium Tin Oxide (ITO). The electrically conductive layer thus formed is patterned to have a predetermined shape, forming the transparent, upper electrodes 107 of the photodiodes 112 along with their connection parts 107a. The electrode 107 is located to cover the step (i.e., the height or level difference) between the layers 104 and 106.
The barrier metal layer 108 is selectively formed on the connection part 107a of the upper electrode 107. The layer 108 is located only in the overlapping area of the part 107a with the overlying signal line 110.
Furthermore, dielectric such as silicon nitride (Si3N4) is deposited on the first interlayer dielectric layer 104 to cover the whole surface of the substrate 101, forming the second interlayer dielectric layer 109. The layer 109 is selectively removed to form the contact holes 118 exposing the barrier metal layer 108 and the contact holes 136 exposing the connection part 105a of the lower electrode 105. At the same time as this, the second and first interlayer dielectric layers 109 and 104 are selectively removed to form the contact holes 137 and 138 exposing respectively the source/drain regions 131a and 131b. 
A metal layer (not shown) is formed on the second interlayer dielectric layer 109 so as to fill the contact holes 118, 136, 137, and 138. The metal layer thus formed is patterned to have a predetermined shape, forming the signal line 110 and the wiring lines 134 and 135 on the layer 109. The metal layer is made of aluminum (Al), for example.
Through the above-described process steps, the prior-art image sensor 100 shown in FIG. 1 is fabricated.
With the prior-art image sensor 100, the photodiode 112 has a layered structure of the lower electrode 105, the a-Si layer 106, and the transparent, upper electrode 107 and at the same time, the upper electrode 107 is formed to cover (or extend through) the step (i.e., the height or level difference) between the first interlayer dielectric layer 104 and the a-Si 106 in order to be electrically connected with the overlying signal line 110. Thus, as shown by the arrow A in FIG. 2, there is a problem that the upper electrode 107 tends to be disconnected or broken near the top edges of the layer 106.
To prevent the break or disconnection of the electrode 107, the thickness of the a-Si layer 106 may be decreased. In this case, however, there arises another problem that the optical absorption rate of the layer 106 tends to lower thereby degrading the sensitivity of the sensor 100 itself.
On the other hand, the Japanese Non-Examined Patent Publication No. 62-204570 published in 1987 discloses an xe2x80x9camorphous silicon (a-Si) image sensorxe2x80x9d, in which the lower electrodes are arranged at intervals on the substrate while a dielectric is formed to fill the recesses or gaps formed between the electrodes. This Publication discloses another structure that the substrate has recesses on its surface and the lower electrodes are formed to be buried in the recesses. Furthermore, it discloses a further structure that a dielectric layer with recesses is formed on the substrate and the lower electrodes are formed to be buried in the recesses.
If any of the structures disclosed in the Publication No. 62-204570 is adopted to the prior-art image sensor 100 described above, no step or height difference occurs in the vicinity of the lower electrode 105 and therefore, the surface of the a-Si layer 106 can be substantially flattened. This is effective for the case where the a-Si layer 106 is not divided into the island to define the pixel areas 121, in other words, the layer 106 is patterned to be linear along the alignment direction of the pixel areas 121.
However, none of the structures disclosed in the Publication No. 62-204570 is able to solve the above-described problem that the upper electrode 107 tends to be broken or disconnected. As already described previously, the break or disconnection of the electrode 107 is caused by the step or height-difference between the a-Si layer 106 and the electrode 107, which is independent of the existence or absence of the lower electrode 105. As a result, the problem of break or disconnection of the electrode 107 is never solved by the use of any of the structures disclosed in the Publication No. 62-204570.
Additionally, with the prior-art image sensor 100 having the island-shaped a-Si layer 106, as shown in FIG. 1, even if some step or height-difference exists in the periphery of the electrode 105, it scarcely affects the operation of the sensor 100 itself.
Moreover, it is typical that the lower electrode 105 has a thickness of approximately 100 nm while the a-Si layer 106 needs to be as thick as approximately 1 xcexcm for sufficient absorption of the incident light. Thus, the step or height-difference in the periphery of the a-Si layer 106 tends to strongly affect the operation of the sensor 100.
Accordingly, an object of the present invention is to provide an image sensor that prevents effectively the break or disconnection of transparent electrodes of photodiodes and a method of fabricating the sensor.
Another object of the present invention is to provide an image sensor that enhances its sensitivity and a method of fabricating the sensor.
Still another object of the present invention is to provide an image sensor that suppresses effectively the smear in image and a method of fabricating the sensor.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to a first aspect of the present invention, an image sensor is provided, which is comprised of:
(a) a transparent substrate;
(b) a first dielectric layer formed over the substrate;
(c) lower electrodes arranged at intervals on the first dielectric layer;
(d) a patterned semiconductor layer formed on the first dielectric layer to overlap with the respective lower electrodes;
(e) transparent upper electrodes formed on the semiconductor layer to overlap with the respective lower electrodes;
(f) a second dielectric layer formed to cover the upper electrodes, the semiconductor layer, and the lower electrodes; and
(g) a patterned signal line layer formed on the second dielectric layer;
the signal line layer being electrically connected in common to the respective upper electrodes at the overlap parts of the semiconductor layer by way of contact holes of the second dielectric layer;
wherein the patterned semiconductor layer has island-shaped pixel parts defining pixel areas of the sensor for receiving incident light through the upper electrodes, overlap parts overlapping with the signal line layer, and interconnection parts interconnecting each of the pixel parts with a corresponding one of the overlap parts;
and wherein each of the upper electrodes, a corresponding one of the pixel parts of the semiconductor layer, and a corresponding one of the lower electrodes constitute a photodiode.
With the image sensor according to the first aspect of the present invention, the lower electrodes are arranged at intervals on the first dielectric layer and the patterned semiconductor layer is formed on the first dielectric layer to overlap with the respective lower electrodes. The upper electrodes are formed on the semiconductor layer to overlap with the respective lower electrodes. The signal line layer is formed on the second dielectric layer that covers the upper electrodes, the semiconductor layer, and the lower electrodes. The signal line layer is electrically connected in common to the respective upper electrodes at the corresponding overlap parts of the semiconductor layer by way of contact holes of the second dielectric layer.
Moreover, the patterned semiconductor layer has the island-shaped pixel parts defining the pixel areas of the sensor for receiving the incident light, the overlap parts overlapping with the signal line layer, and the interconnection parts interconnecting each of the pixel parts with a corresponding one of the overlap parts. Each of the upper electrodes, the corresponding one of the pixel parts of the semiconductor layer, and the corresponding one of the lower electrodes constitute the photodiode.
Accordingly, the transparent upper electrodes are not to contact the top edges of the respective pixel parts of the underlying semiconductor layer, in other words, the upper electrodes extend toward the signal line layer without penetrating through the vicinity of the top edges of the pixel parts of the semiconductor layer. Thus, the break or disconnection of the transparent upper electrodes of the photodiodes can be effectively prevented.
Also, since the break or disconnection of the upper electrodes can be effectively prevented, the thickness of the semiconductor layer can be sufficiently large. This means that the sensitivity of the image sensor can be enhanced.
In a preferred embodiment of the sensor according to the first aspect, a patterned light-shielding layer is additionally formed between the semiconductor layer and the substrate. The light-shielding layer serves to prevent light having entered through the substrate from reaching the semiconductor layer. In this embodiment, there is an additional advantage that the smear in image due to the fact that the overlap and/or interconnection parts of the semiconductor layer absorb the light having entered through the substrate can be suppressed effectively.
In this embodiment, preferably, the light-shielding layer shields the light having entered through the substrate not to enter the interconnection parts and the overlap parts of the semiconductor layer. In other words, the light-shielding layer is formed to entirely overlap with (or entirely cover) the interconnection parts and the overlap parts of the semiconductor layer.
In this embodiment, it is preferred that the light-shielding layer is electrically connected in common to the upper electrodes. This is to prevent the combination of the light-shielding layer, the overlap and/or interconnection parts of the semiconductor layer, and the upper electrodes from operating as photodiodes.
In another preferred embodiment of the sensor according to the first aspect, a patterned light-shielding layer is additionally formed between the semiconductor layer and the substrate. The light-shielding layer serves to prevent an unwanted carrier generated in the interconnection parts of the semiconductor layer from diffusing into the pixel parts of the semiconductor layer. The unwanted carrier is generated by absorption of light having entered through the substrate in the interconnection parts of the semiconductor layer.
In this embodiment, preferably, the light-shielding layer has a width equal to sum of a diffusion length of the unwanted carrier and a width of an overlapping area of the light-shielding layer with the respective lower electrodes.
In this embodiment, it is preferred that the light-shielding layer is electrically connected in common to the upper electrodes. This is to prevent the combination of the light-shielding layer, the overlap and/or interconnection parts of the semiconductor layer, and the upper electrodes from operating as photodiodes.
In still another preferred embodiment of the sensor according to the first aspect, a first, patterned light-shielding layer is additionally formed between the semiconductor layer and the first interlayer dielectric layer and at the same time, a second, patterned light-shielding layer is additionally formed between the first interlayer dielectric layer and the substrate. The first light-shielding layer is apart from the respective lower electrodes by gaps. The second light-shielding layer is located to cover the gaps. The first and second light-shielding layers jointly serve to prevent light having entered through the substrate from reaching the overlap and interconnection parts of the semiconductor layer.
In this embodiment, preferably, the second light-shielding layer is located to overlap with the first light-shielding layer and the respective lower electrodes.
In this embodiment, it is preferred that the first and second light-shielding layers are electrically connected in common to the upper electrodes. This is to prevent the combination of the first and second light-shielding layers, the overlap and/or interconnection parts of the semiconductor layer, and the upper electrodes from operating as photodiodes.
According to a second aspect of the present invention, a method of fabricating an image sensor is provided. This method, which is applicable to fabrication of the image sensor according to the first aspect, comprises the steps of:
(a) forming a first dielectric layer over a transparent substrate;
(c) forming lower electrodes to be arranged at intervals on the first dielectric layer;
(d) forming a patterned semiconductor layer on the first dielectric layer to overlap with the respective lower electrodes;
(e) forming transparent upper electrodes on the semiconductor layer to overlap with the respective lower electrodes;
(f) forming a second dielectric layer to cover the upper electrodes, the semiconductor layer, and the lower electrodes; and
(g) forming a patterned signal line layer on the second dielectric layer in such a way that the signal line layer is electrically connected in common to the respective upper electrodes at the overlap parts of the semiconductor layer by way of contact holes of the second dielectric layer;
wherein in the step (d), the patterned semiconductor layer is formed to have island-shaped pixel parts defining pixel areas of the sensor for receiving incident light through the upper electrodes, overlap parts overlapping with the signal line layer, and interconnection parts interconnecting each of the pixel parts with a corresponding one of the overlap parts;
and wherein each of the upper electrodes, a corresponding one of the pixel parts of the semiconductor layer, and a corresponding one of the lower electrodes constitute a photodiode.
With the method of fabricating an image sensor according to the second aspect of the present invention, because of substantially the same reason as the sensor according to the first aspect, the same advantages as those in the sensor according to the first aspect are given.